In prior art, a wrapping machine as defined in the preamble is known from specification U.S. Pat. No. 5,875,616.
Specification U.S. Pat. No. 5,875,616 discloses a wrapping machine for winding a wrapping film web around an object to be packaged. The wrapping machine comprises a film dispenser, which comprises a frame. The frame is provided with supporting elements for rotatably supporting a film web roll on the frame.
The film dispenser comprises a pre-stretching device connected to the frame for pre-stretching the film web drawn from the film web roll before its being wrapped around the object. The pre-stretching device comprises a first pre-stretching roller, with the outer surface of whose rim the first side of the film web drawn from the roller can be brought into a tractive frictional contact. The pre-stretching device further comprises a second pre-stretching roller mounted with bearings on the frame so as to be rotatable at a distance from the first pre-stretching roller to form a first gap between them. The second side of the film web drawn from the roller can be brought into a tractive frictional contact with the outer surface of the rim of the second pre-stretching roller. The pre-stretching device further comprises a first transmission gear, which is mounted on the shaft of the first pre-stretching roller, and a second transmission gear, which is mounted on the shaft of the second pre-stretching roller and fitted to be in tractive contact with the first transmission gear to produce a transmission ratio so as to differentiate the circumferential velocities of the first and second pre-stretching rollers.
The film dispenser further comprises a first deflecting pinch roller, which is fitted to be pressed against the first pre-stretching roller to form a first nip for the film web passed between the first deflecting pinch roller and the first pre-stretching roller. In addition, the film dispenser comprises a second deflecting pinch roller, which is fitted to be pressed against the second pre-stretching roller to form a second nip for the film web passed between the second deflecting pinch roller and the second pre-stretching roller.
The frame of the film dispenser according to specification U.S. Pat. No. 5,875,616 is divided into two sections, a fixed section and an openable and closeable gate section pivotally connected to it and forming a sort of lever mechanism, the first pre-stretching roller and one of the deflecting pinch rollers being mounted with bearings on the gate section. The second pre-stretching roller and the second deflecting pinch roller are rotatably mounted with bearings on the fixed frame. The section part can be moved between an engagement position and a disengagement position. In the engagement position, the power transmission wheels of the pre-stretching rollers are in driving engagement with each other. In the disengagement position, when the gate section is open, the power transmission wheels are disengaged from each other. In the disengagement position with the gate section open, the first gap between the pre-stretching rollers is naturally larger than the corresponding first gap in the engagement position. Due to the openable gate section, the feed-in of the end of the film web from the film web roll through such a pre-stretching device can be easily implemented.
A problem with this prior-art solution is, however, that the film dispenser provided with an openable gate section is fairly big and heavy. In addition, the structure is expensive and relatively complicated.
To solve the problem relating to the feed-in of the film end, other solutions have also been developed, e.g. as disclosed in specification EP 1019288 B1. The ends of the pre-stretching rollers are provided with tapered extensions allowing the film web end crinkled into the form of a narrow strip to slide over the extensions, which guide the strip onto the pre-stretching rollers.
A problem with the structure according to this EP patent is that the extensions of the rollers increase the size of the film dispenser in the vertical direction.